feat: add OpenTelemetry tracing support behind otel feature flag

- Add tracing::instrument spans to executor (workflow.execute),
  host (workflow.start, event.publish, event.process)
- Add otel feature flag to wfe-core and wfe crates
- Add wfe/src/otel.rs helper for OTLP exporter initialization
- Dependencies: tracing-opentelemetry, opentelemetry, opentelemetry_sdk,
  opentelemetry-otlp (all optional behind otel feature)
- Step execution stays at info level, executor internals at debug

wfe-core/Cargo.toml

@@ -8,8 +8,11 @@ description = "Core traits, models, builder, and executor for the WFE workflow e
 [features]
 default = []
 test-support = []
+otel = ["tracing-opentelemetry", "opentelemetry"]
 
 [dependencies]
+tracing-opentelemetry = { workspace = true, optional = true }
+opentelemetry = { workspace = true, optional = true }
 tokio = { workspace = true }
 serde = { workspace = true }
 serde_json = { workspace = true }